Protective mechanisms of nitrogenase against oxygen excess and partially‐reduced oxygen intermediates

Diazotrophic systems have developed a number of strategies to protect nitrogenase (N 2 ase; EC 1.18.6.1) from O 2 excess and active‐oxygen species (AOS). Protection against O 2 excess is given by biochemical modifications of N 2 ase, increased rates of low‐efficiency respiration, temporal segregation of N 2 fixation and photosynthesis, physical barriers to O 2 diffusion, and hemoglobins. On the other hand, AOS may originate from oxidation of N 2 ase components, ferredoxins, flavodoxins and hemoglobins; interaction among the AOS themselves, or between H 2 O 2 and hemoglobins; and during reactions catalyzed by hydrogenase (EC 1.18.99.1), xanthine oxidase (EC 1.1.3.22) and uricase (EC 1.7.3.3). Active‐oxygen species are scavenged enzymatically [superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6). peroxidase (EC 1.11.1.7), ascorbate peroxidase (EC 1.11.1.11)] or through non‐enzymic reaction with low‐molecular‐weight compounds (ascorbate, α‐tocopherol, glutathione).